We can choose a reference type (Composition/Parent-child/Weak reference). I think "Composition/Parent-child" mean "1:n" and "Weak reference" means "n:n". What difference between Composition and Parent child?
From the manual
Parent-Child - This is a strong reference type in which an element exists as long as it has one ore more strongly related parents or belongs to a composition relationship.
Composition - In this relationship, the parent element solely defines and owns the child element. The child can be contained only in one composition parent relationship and deletion of the parent also deletes the child.
Weak - This is a weak reference type in which the parent does not define the lifetime of the child.
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